* Fix r111983 (bug 34662): make mw.toolbar.addButton() work even after DOM ready
* Don't use an empty array as the default for $toolbar, use false which will throw an exception on a logic error.
* Fix exception handling in mw.loader: have handlePending() rethrow its exception so that the exception handler in execute() can run, so that the exception gets logged. But don't rethrow the exception in execute(), because that may break other modules.